The present invention relates in general to transducers for musical instruments and pertains, more particularly, to a bridge mounted transducer array for a stringed instrument such as a bass violin.
The bass violin, when played in a popular or jazz group is plucked rather than bowed. In this mode of operation the instrument is rather soft in volume and must be electronically amplified. Simply placing a microphone in front of the instrument on a stage is generally not satisfactory because the microphone tends to pick up other sounds on the stage (drums, etc.). In addition, when the microphone is turned up high enough to pick up the sounds of this quiet instrument, the microphone is very prone to acoustical feedback or squealing.
A more successful approach to this amplification problem has been to place a vibration sensitive transducer directly on the instrument itself. This type of pick-up senses the vibrations on the instrument body only, and is not affected by the ambient vibrations. This style of pick-up is commonly referred to as a contact pick-up. The contact pick-ups generally available are usually a plug of piezoceramic material encased in epoxy or plastic to protect the fragile crystal. This piezoceramic crystal has the piezoelectric property of emitting a d.c. voltage when deformed. When mounted on the instrument (usually with tape or putty) the vibrations of the instrument bend the crystal which, in turn, emits an electrical analog of the mechanical vibration. This electrical signal can then be amplified and used to drive loudspeakers.
Although the contact type pick-up is an improvement over the microphone, it still has inherent problems. The placement on the instrument is critical so as to provide an even response from each string. At high volumes, when the pick-up is improperly located, it causes the entire instrument to become microphonic and thus create a feed-back similar to the type of feed-back experienced with a stage microphone. Most of the transducers available have been unsuccessful in properly reproducing the characteristic sound of the instrument. Their inability to do so generally arises from improper location on the instrument and associated mounting technique which is often responsible for the less than optimum sensing location of the transducer. An example of a commonly used, but rather undesirable method of mounting is securing the transducer element to the vibratile member with a layer of wax or putty. However, it has been found that the mechanical coupling provided by these compressible materials is too resilient to accurately transfer the vibrations from the instrument body to the transducer.
In accordance with the invention it has been found that as far as the location of the transducers is concerned, the bridge rather than the body of the instrument is preferred due to the improved transient response that is available when the transducers are mounted physically close to the initial vibration producer (the string). The general objective is to provide the characteristic acoustic sound. In accordance with the invention it is an object to determine the best location to capture this sound at the same time eliminating feedback. In accordance with the invention the transducer element and its associated mounting are designed to operate in this optimized location. In this connection experiments have been made to better define the different types of vibrations that are present on the bridge to ascertain a proper location. A full scale drawing of the bridge is made on one-quarter inch square grid paper. A point source piezoelectric transducer one-quarter inch by one-quarter inch is used. This point source transducer is placed at various points on the bridge and the instrument is then played. The location is then noted on the grid and the sound is graded by the player with regard to the following criteria:
1. Pure string tone (sinusoidal).
2. Transient response--attack.
3. Nodally enhanced tone.
4. Feedback resistance.
One may define particular areas on the bridge and analyze them. In this regard reference can now be made to FIG. 3 wherein areas A, B, C and D are defined. Area A provides a pure sound strongest in pure sinusoidal string tone with good transient response and high feedback resistance. Area B is a transition area that is mostly pure but has some nodal presence with good feedback resistance and good attack. Area C which lies generally below areas A and B is highly nodal, very unstable, has little pure sound, has poor attack and a high feedback. Area D which essentially surrounds each string has the best attack.
The characteristic acoustic sound of the instrument is generally determined by the manner in which the resonator enhances and modifies the pure tone produced by the vibrating string. Thus, it is desired to have a pick-up that can sense both pure string vibrations and enhanced vibrations. In other words it is desirable to have a pick-up that can sense various degrees of the pure tone and enhanced areas in addition to the impulse and low frequency areas. In accordance with the present invention it has been found that a pair of transducers is generally preferred over the use of four separate transducers. In this regard, one transducer and element senses the impulse areas of the g and d strings while the other transducer senses the impulses of the a and e strings. In addition, each of the transducers is preferably arranged to cover area b referred to hereinbefore, the transition area.
Another feature in accordance with the present invention is the adjustability of the transducer arrangement. In some instruments, one string may be often found to be less responsive than the other strings. This imbalance can be adjusted in accordance with the present arrangement by sensing more of that string's impulse area. Thus, the pick-ups can be easily moved to adjust the sensing. Also, the pick-ups can be easily moved up or down to sense different proportions of pure or enhanced sound. The overall sound can be adjusted to the taste of the performer without the necessity of elaborate signal equipment at the amplifier end.
Also, in accordance with the invention it has been found that the transducer element is preferably extremely compliant so as to accurately discriminate the individual areas it covers. A wafer thin piezoelectric crystal used alone is generally too fragile. Thus, in accordance with the invention there is provided a combination of crystal laminated to a soft metal such as brass. With this arrangement, tracking has been found to be excellent and the transducer is also quite durable. In accordance with the invention attachment via a felt padded brass clip provides easy mounting, adjustability, and added protection to the crystal.
Existing bridge-mounted devices generally operate in an area on the instrument which is far less than optimum. For example, the typical putty-mounted transducers, because of their poor tracking ability, only operate in areas of large magnitude vibrations. These areas only exist in the undamped sections of the bridge which are full of undesirable harmonics. Another example of a device presently used is a transducer that is wedged into wing-slots on the bridge. These devices track better than the putty mounted transducers but still sense harmonics present in the wings. They also suffer from a frequency-doubling effect which tends to produce first octave rather than fundamental emphasis. This effect is not true to the characteristic acoustic sound. See for example the Underwood U.S. Pat. No. 4,147,084.
Accordingly, one object of the present invention is to provide a bridge mounted pick-up that is readily mounted in a highly damped, node-free area of the bridge and that is sufficiently sensitive to accurately produce an analog of the vibrations of the mounting area.
Another object of the present invention is to provide a pick-up that is readily mounted to the stringed instrument without modification of the instrument.
Still another object of the present invention is to provide an improved transducer design that is extremely feedback resistant even at very high volume.
A further object of the present invention is to provide an improved means of mounting a transducer in association with a stringed musical instrument. In accordance with this object, the transducer or transducers are mounted close to the string to provide a short transient response resulting in improved "attack" and more "presence".
Still a further object of the present invention is to provide an improved transducer design and associated means for mounting associated with a stringed musical instrument and characterized by a greater fundamental pitch definition. This improved definition results by placing the transducers at the point of optimum damping. At other locations the elements tend to artificially emphasize nodes which are often present.